I've started my PPL lessons and have so far done 35 hours, so unfortunately my experience is limited!
At A-level I did Maths, Physics, Biology and, for the first year, Computing. I then did a degree in Maths.
I would say that out of all my subjects, A-Level Physics has been by far the most useful. The Mathematics you study at A level reaches far beyond what a pilot is required to do, (you're not exactly going to be sat in your cockpit trying to figure out a very difficult Mathematical equation). That said, I would still recommend doing Maths if that's what you fancy. The more Mathematically fluent you are, the better!
As for French, I was under the impression that all communication done in Aviation is in English. If this is not the case then I bow to superior knowledge!
As for anything else, you will be required to learn it as part of your pilot examinations anyway, so what you choose at A level isn't overly important!
Hope that helps